US11225701B2 - Hot dip galvanized steel sheet and hot dip galvannealed steel sheet - Google Patents

Hot dip galvanized steel sheet and hot dip galvannealed steel sheet Download PDF

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Publication number
US11225701B2
US11225701B2 US17/043,267 US201817043267A US11225701B2 US 11225701 B2 US11225701 B2 US 11225701B2 US 201817043267 A US201817043267 A US 201817043267A US 11225701 B2 US11225701 B2 US 11225701B2
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steel sheet
hot dip
γgb
less
temperature
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US20210017622A1 (en
Inventor
Jun Haga
Kohichi Sano
Koutarou Hayashi
Kunio Hayashi
Masaharu Kameda
Akihiro Uenishi
Hiroyuki Kawata
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Nippon Steel Corp
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Nippon Steel Corp
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: SANO, KOHICHI, HAGA, JUN, HAYASHI, KOUTAROU, HAYASHI, KUNIO, KAMEDA, MASAHARU, KAWATA, HIROYUKI, UENISHI, AKIHIRO
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B32B15/043—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material of metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/18—Layered products comprising a layer of metal comprising iron or steel
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
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    • C23C2/26—After-treatment
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    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
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    • C23C28/3225—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer only coatings of metal elements only with at least one zinc-based layer
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • C23C30/005—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process on hard metal substrates
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y10T428/12979—Containing more than 10% nonferrous elements [e.g., high alloy, stainless]

Definitions

  • the present invention relates to high strength hot dip galvanized steel sheet and high strength hot dip galvannealed steel sheet suitable for press-forming, more particularly relates to high strength hot dip galvanized steel sheet and high strength hot dip galvannealed steel sheet excellent in ductility and low temperature impact property.
  • the steel sheet has to have excellent impact absorbing property.
  • the steel sheet for automotive use has to be higher in strength and also excellent in local ductility so as to suppress cracking at the time of application of an impact load.
  • steel sheet for automotive use is being required to be high in strength for lightening the weight of auto bodies and improving collision safety, high uniform ductility for improving shapeability, and high local ductility for improving shapeability and improving collision safety. Furthermore, to secure collision safety even in low temperature environments, steel sheet for automotive use is being required to be excellent in low temperature impact property.
  • steel sheet improvement of uniform ductility and local ductility and higher strength and improvement of low temperature impact property and higher strength are both contradictory matters. It is difficult to simultaneously achieve these. Further, steel sheet for automotive use is being required to be corrosion resistant, but to maintain the required corrosion resistance, securing ductility and a low temperature impact property while raising the strength is made further difficult.
  • PTLs 1 and 2 disclose methods for manufacturing high strength cold rolled steel sheet comprising heating steel sheet containing Si and Mn to the ferrite-austenite dual phase region or the austenite single phase region to anneal it, then cooling it, then holding the steel sheet at 350 to 500° C. to austemper it and stabilize the austenite. According to these manufacturing methods, in cold rolled steel sheet, it is possible to improve the strength and ductility with a good balance.
  • PTL 3 discloses a method of manufacturing high strength hot dip galvannealed steel sheet comprising including certain ratios of more of Si and Mn with respect to the amount of C to thereby suppress transformation of austenite during alloying and form a metal structure in which retained austenite is mixed with ferrite.
  • PTL 4 discloses high strength hot dip galvanized steel sheet comprised of average grain size 10 ⁇ m or less ferrite and tempered martensite into which retained austenite and phases formed by low temperature transformation are dispersed so as to realize excellent ductility, flange elongation, and fatigue resistance. Further, it discloses that tempered martensite is a phase effective for improving the flange elongation and fatigue resistance and that if refining the tempered martensite, the above properties are further improved.
  • the present invention was made in consideration of the prior art and has as its technical issue the provision of hot dip galvanized steel sheet and hot dip galvannealed steel sheet in which hot dip galvanized steel sheet and hot dip galvannealed steel sheet the uniform ductility and local ductility and the low temperature impact property and yield strength and tensile strength are all improved.
  • the inventors studied in depth methods for solving the above technical issue. As a result, they obtained the findings of the following (A) to (D).
  • the inventors learned based on the findings of (A) to (D) that if temper rolling hot dip galvanized steel sheet or hot dip galvannealed steel sheet, then again heat treating it by two-stage heating, it is possible to manufacture hot dip galvanized steel sheet and hot dip galvannealed steel sheet having a metal structure containing high C concentration retained austenite and tempered martensite, excellent in uniform ductility, local ductility, and low temperature impact property and, furthermore, high in yield strength and tensile strength.
  • steel sheet includes “steel strip”.
  • a chemical composition of the steel sheet comprises, by mass %, at least one of B: 0.0002 to 0.0200%, Ti: 0.001 to 0.30%, Nb: 0.001 to 0.30%, V: 0.001 to 0.30%, Cr: 0.001 to 2.00%, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Ca: 0.0001 to 0.010%, Mg: 0.0001 to 0.010%, REM: 0.0001 to 0.10%, and Bi: 0.0001 to 0.050%.
  • a method of producing the hot dip galvanized steel sheet according to any one of (1) to (3) comprising an annealing step of heating a base steel sheet having a chemical composition as described in (1) or (2) to a temperature region over an Ac 1 point to anneal it, a first cooling step of cooling the base steel sheet after the annealing step down to 500° C. or less by an average cooling rate at a temperature region of 650 to 500° C. of 2° C./s or more and less than 100° C./s, a plating step of hot dip galvanizing the base steel sheet after the first cooling step, a second cooling step of cooling the base steel sheet after the plating step down to less than 300° C.
  • a method of producing a hot dip galvannealed steel sheet according to (4) comprising an annealing step of heating a base steel sheet of a chemical composition as described in (1) or (2) to a temperature region over an Ac 1 point to anneal it, a first cooling step of cooling the base steel sheet after the annealing step down to 500° C. or less by an average cooling rate at a temperature region of 650 to 500° C. of 2° C./s or more and less than 100° C./s, a plating step of hot dip galvanizing the base steel sheet after the first cooling step, an alloying step of alloying the base steel sheet after the plating step, a second cooling step of cooling the base steel sheet after the alloying step down to less than 300° C.
  • the present invention it is possible to provide hot dip galvanized steel sheet and hot dip galvannealed steel sheet in which the uniform ductility and local ductility are both excellent, which are excellent in press-formability, further in which the yield strength and tensile strength are high and local ductility is excellent, which are excellent in impact absorbing property, and furthermore which are also excellent in low temperature impact property.
  • the steel sheet of the present invention will be successively explained.
  • C is an element required for obtaining retained austenite. Furthermore, it is an element strengthening the grain boundaries by segregation at the prior austenite grain boundaries in the steel sheet of the present invention. If C is less than 0.03%, it becomes difficult to obtain a metal structure containing retained austenite and tempered martensite, so C is made 0.03% or more. Preferably, it is 0.10% or more, more preferably 0.13% or more, still more preferably 0.16% or more.
  • C is made 0.70% or less.
  • it is 0.30% or less, more preferably 0.26% or less, still more preferably 0.24% or less.
  • Si is an element acting to suppress the segregation of cementite and promote the formation of retained austenite. Further, it is an element keeping the tempered martensite from excessively softening and contributing to securing strength.
  • Si is less than 0.25%, the effect of addition is not sufficiently obtained, so Si is made 0.25% or more, preferably over 0.60%, more preferably over 1.00%, still more preferably over 1.45%.
  • Si is made 2.50% or less.
  • it is 2.30% or less, more preferably 2.10% or less, still more preferably 1.90% or less.
  • Mn is an element contributing to improvement of the hardenability of steel and effective for obtaining a metal structure containing retained austenite and tempered martensite. If Mn is less than 1.00%, the effect of addition is not sufficiently obtained, so Mn is made 1.00% or more. Preferably, it is over 1.50%, more preferably over 2.00%, still more preferably over 2.50%.
  • Mn is made 5.00% or less.
  • Mn is 4.00% or less, more preferably 3.50% or less, still more preferably 3.00% or less.
  • P is an element which segregates at the prior austenite grain boundaries to make the steel sheet brittle, so the less the better.
  • the present invention is art suppressing segregation of P at the prior austenite grain boundaries and causing segregation of C and B and is predicated on P remaining in the steel to a certain extent. For that reason, there is no need to excessively reduce P. In particular, if reducing P to less than 0.0005%, the manufacturing cost greatly rises, so P can be made 0.0005% or more. It may also be made 0.0010% or more.
  • P is made 0.100% or less.
  • it is made less than 0.020%, more preferably less than 0.015%, still more preferably less than 0.010%.
  • S is an element forming sulfide-based inclusions in the steel for obstructing local ductility of the steel sheet, so the less the better. If S is over 0.010%, the local ductility of the steel sheet remarkably falls, so S is made 0.010% or less. Preferably, it is 0.0050% or less, more preferably 0.0012% or less.
  • the lower limit includes 0%, but if reducing S to less than 0.0001%, the manufacturing cost greatly rises, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • Al like Si, is an element acting to deoxidize the molten steel. Further, it is an element promoting the formation of retained austenite and effective for forming a metal structure containing retained austenite and tempered martensite.
  • sol. Al is made 0.001% or more.
  • it is 0.015% or more, more preferably 0.025% or more, still more preferably 0.045% or more.
  • it is preferably 0.055% or more, more preferably 0.065% or more, still more preferably 0.075% or more.
  • sol. Al is made 2.500% or less. Preferably, it is less than 0.600%, more preferably less than 0.200%, still more preferably less than 0.080%.
  • N is an element forming nitrides causing cracks in a slab during continuous casting of steel, so the less, the better. If N is over 0.020%, slab cracks frequently occur, so N is made 0.020% or less. Preferably, it is 0.010% or less, more preferably less than 0.008%, still more preferably 0.005% or less.
  • the lower limit includes 0%, but if reducing N to less than 0.0005%, the manufacturing costs greatly rise, so in practical steel sheet, 0.0005% is the substantive lower limit.
  • B is an element which segregates at the prior austenite grain boundaries and strengthens the grain boundaries.
  • B is an element improving the hardenability of steel and effective for forming a metal structure containing retained austenite and tempered martensite. To sufficiently obtain the effect of addition, B is preferably made 0.0002% or more. More preferably, it is 0.0005% or more, still more preferably 0.0010% or more.
  • B is made 0.0200% or less.
  • it is 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020% or less.
  • the steel sheet of the present invention may include, in addition to the above elements, one or more of Ti, Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, and Bi so as to improve the properties.
  • Ti, Nb, and V are elements refining the metal structure and contributing to improvement of the strength and ductility of the steel sheet.
  • Ti, Nb, and V are all preferably 0.001% or more. More preferably, Ti and Nb are 0.005% or more and V is 0.010% or more, still more preferably Ti and Nb are 0.010% or more and V is 0.020% or more.
  • Ti, Nb, and V exceed 0.30%, the effect of addition becomes saturated and the recrystallization temperature at the time of annealing rises resulting in the annealed metal structure becoming uneven and the local ductility falling, so Ti, Nb, and V all are 0.30% or less. More preferably, Ti is less than 0.080%, Nb is less than 0.050%, and V is 0.20% or less, still more preferably Ti is 0.035% or less, Nb is 0.030% or less, and V is less than 0.10%.
  • Cr and Mo are elements improving the hardenability of steel and contributing to formation of a metal structure containing retained austenite and tempered martensite. To sufficiently obtain the effects of addition of Cr and Mo, both are preferably 0.001% or more. More preferably, Cr is 0.100% or more, while Mo is 0.050% or more.
  • Cr and Mo are both 2.00% or less. More preferably, Cr is 1.00% or less and Mo is 0.50% or less.
  • Cu and Ni are elements contributing to improvement of yield strength and tensile strength. To sufficiently obtain the effects of addition of Cu and Ni, Cu and Ni are both preferably 0.001% or more. More preferably, both elements are 0.010% or more.
  • Cu and Ni exceed 2.00%, the effect of addition becomes saturated and the economy falls, so Cu and Ni are both 2.00% or less. More preferably, both elements are 0.80% or less.
  • Ca, Mg, and REM are elements controlling the shape of inclusions to contribute to improvement of the local ductility.
  • Ca, Mg, and REM are all preferably 0.0001% or more. More preferably, each element is 0.0005% or more.
  • Ca and Mg are over 0.010%, the effect of addition becomes saturated and the economy falls, so Ca and Mg are preferably 0.010% or less. More preferably each element is 0.002% or less.
  • the REM is made 0.10% or less. More preferably, it is 0.010% or less.
  • REM is the general name for Sc, Y, and lanthanoids: a total of 17 elements. Lanthanoids are industrially added in the form of mischmetal. Note that the amount of the REM is the total amount of these elements.
  • Bi is an element refining the solidified structure to contribute to improvement of the local ductility.
  • Bi is preferably 0.0001% or more. More preferably, it is 0.0003% or more.
  • Bi is preferably 0.050% or less. More preferably it is 0.010% or less, still more preferably 0.005% or less.
  • the balance of the chemical composition of the steel sheet of the present invention is Fe and unavoidable impurities.
  • the unavoidable impurities are elements unavoidably entering from the steel raw material (ore, scrap, etc.) and/or at the manufacturing process and are elements allowed in a range not obstructing the properties of the steel sheet of the present invention.
  • the metal structure of the steel sheet of the present invention will be explained.
  • the “%” relating to the structural percentage means the “vol %”.
  • the metal structure of the steel sheet of the present invention is a metal structure comprised of, by vol %, retained austenite of over 5.0% and tempered martensite of over 5.0%. By forming this metal structure, it is possible to maintain the yield strength and tensile strength while improving the uniform ductility and local ductility.
  • the retained austenite is 5.0% or less, the uniform ductility is not improved, so the retained austenite is made over 5.0%. Preferably, it is over 6.0%, more preferably over 8.0%, still more preferably over 10.0%.
  • vol % of the retained austenite is not unambiguously determined in relation to the vol % of other structures, so no upper limit can be set, but if 30.0% or more, the local ductility and low temperature impact property deteriorate, so the retained austenite is preferably less than 30.0%. More preferably, it is less than 20.0%.
  • the tempered martensite is 5.0% or less, it is difficult to maintain the yield strength and tensile strength while raising the local ductility, so the tempered martensite is made over 5.0%. Preferably, it is over 8.0%, more preferably over 10.0%, still more preferably over 12.0%.
  • the vol % of the tempered martensite is not unambiguously determined in relation to the vol % of other structures, so no upper limit can be set, but if 70.0% or more, the uniform ductility falls, so the tempered martensite is preferably 70.0% or less. More preferably, it is 50.0% or less, still more preferably 30.0% or less.
  • the balance of the metal structure is a structure including polygonal ferrite, low-temperature transformation structure such as martensite (indicating not tempered martensite, also called “fresh martensite”), acicular ferrite or bainite, and pearlite, and furthermore precipitates such as cementite and others.
  • martensite indicating not tempered martensite, also called “fresh martensite”
  • acicular ferrite or bainite acicular ferrite or bainite
  • pearlite furthermore precipitates such as cementite and others.
  • Polygonal ferrite is a structure effective for raising the uniform ductility, so is preferably contained in over 2.0%. More preferably, it is 3.0% or more.
  • the vol % of the polygonal ferrite is not unambiguously determined in relation to the vol % of other structures, so no upper limit can be set, but if the polygonal ferrite is 50.0% or more, the yield strength and the tensile strength fall and furthermore the local ductility falls, so the polygonal ferrite is preferably less than 50.0%. More preferably, it is less than 20.0%, still more preferably less than 10.0%.
  • Martensite is a structure obstructing the maintenance of yield strength and improvement of the local ductility, so the less, the better. Preferably, it is less than 5.0%. More preferably, it is less than 2.0%, still more preferably less than 1.0%.
  • Low-temperature transformation structure such as acicular ferrite or bainite, and precipitates such as pearlite, cementite, and others obstruct the yield strength and tensile strength, so the total is preferably 40.0% or less. More preferably, it is 20.0% or less, still more preferably 10.0% or less.
  • Pearlite inhibits the uniform ductility in addition to the yield strength and tensile strength, so less than 10.0% is preferable. More preferably, it is less than 5.0%, still more preferably less than 3.0%.
  • the low-temperature transformation structure such as martensite, acicular ferrite, and bainite, pearlite, and precipitates such as cementite and others are sometimes unavoidably formed, so lower limits are not particularly prescribed, but the balance of the metal structure does not have to include these structures, so the lower limit is 0%.
  • vol % of the metal structure of the steel sheet of the present invention is measured in the following way.
  • a test piece was taken from the steel sheet, a longitudinal section parallel to the rolling direction was polished, then the metal structure at a depth position of 1 ⁇ 4 of the sheet thickness of the base steel sheet from the boundary of the base steel sheet and plating layer was observed and photographed by a scan electron microscope (SEM). The image was processed to calculate the area ratios of the structures and the calculated area ratios were made the volume ratios.
  • Tempered martensite can be differentiated from bainite on the point that iron carbides present inside it extend in a plurality of directions.
  • Polygonal ferrite can be differentiated from acicular ferrite on the point that it is clump shaped in form and on the point that the dislocation density is low.
  • the amount of C of the retained austenite is made 0.85 mass % or more. Preferably, it is 0.87 mass % or more, more preferably 0.89 mass % or more. Note that, the “amount of C of the retained austenite” means the concentration of C at the austenite phases.
  • the amount of C of the retained austenite fluctuates according to the amount of C of the steel sheet and the manufacturing conditions, so the upper limit cannot be set, but if the amount of C is 1.50 mass % or more, the TRIP effect cannot be obtained and the uniform ductility falls, so the amount of C of the retained austenite is preferably less than 1.50 mass %. More preferably, it is less than 1.20 mass %, still more preferably less than 1.10 mass %.
  • vol % of the retained austenite and the amount of C of the retained austenite are calculated by chemical polishing the rolling surface at a depth position of 1 ⁇ 4 of the sheet thickness of the base steel sheet from the boundary of the base steel sheet and plating layer at the test piece taken from the steel sheet and measuring the X-ray diffraction intensity and diffraction peak position of the polished surface by an X-ray diffraction apparatus (XRD).
  • XRD X-ray diffraction apparatus
  • [C] ⁇ gb /[P] ⁇ gb is less than 4.0, the low temperature impact property is not improved, so [C] ⁇ gb /[P] ⁇ gb is made 4.0 or more.
  • it is 5.0 or more, more preferably 6.0 or more.
  • the upper limit is not particularly prescribed, but from the viewpoint of productivity, 30.0 or less is preferable.
  • the [C] ⁇ gb and [P] ⁇ gb at the prior austenite grain boundaries are measured in the following way to calculate [C] ⁇ gb /[P] ⁇ gb .
  • the metal structure at a depth position of 1 ⁇ 4 of the sheet thickness of the base steel sheet from the boundary of the base steel sheet and the plating layer is observed to confirm the prior austenite grain boundaries.
  • the lift-out method is used to cut out a block including the prior austenite grain boundaries and a focused ion beam apparatus (FIB) is used to prepare a pin sample for use as a three-dimensional atom probe (3DAP).
  • FIB focused ion beam apparatus
  • the distributions of atoms of C and P at the region including the prior austenite grain boundaries are measured.
  • the distributions of concentration are analyzed by a ladder diagram to find the densities of segregated atoms per unit grain boundary area (interfacial excess).
  • the obtained values are made [C] ⁇ gb and [P] ⁇ gb .
  • [C] ⁇ gb and [P] ⁇ gb are measured for different prior austenite grain boundaries of five or more locations.
  • the average value of the values of the [C] ⁇ gb /[P] ⁇ gb obtained at the prior austenite grain boundaries are found.
  • the steel sheet of the present invention contains B, furthermore, by making the ratio [B] ⁇ gb /[P] ⁇ gb of the amount of segregation of B (number of atoms/nm 2 ): [B] ⁇ gb at the prior austenite grain boundaries to the amount of segregation of P (number of atoms/nm 2 ): [P] ⁇ gb at the prior austenite grain boundaries 4.0 or more, the low temperature impact property is remarkably improved.
  • [B] ⁇ gb /[P] ⁇ gb is less than 4.0, the low temperature impact property is not improved, so [B] ⁇ gb /[P] ⁇ gb is made 4.0 or more.
  • it is 5.0 or more, more preferably 6.0 or more.
  • the upper limit is not particularly prescribed, but from the viewpoint of productivity, 30.0 or less is preferable.
  • the [B] ⁇ gb and [P] ⁇ gb at the prior austenite grain boundaries are measured in the following way to calculate [B] ⁇ gb /[P] ⁇ gb .
  • the metal structure at a depth position of 1 ⁇ 4 of the sheet thickness of the base steel sheet from the boundary of the base steel sheet and the plating layer is observed to confirm the prior austenite grain boundaries.
  • the lift-out method is used to cut out a block including the prior austenite grain boundaries and a focused ion beam apparatus (FIB) is used to prepare a pin sample for use as a three-dimensional atom probe (3DAP).
  • FIB focused ion beam apparatus
  • the distributions of atoms of B and P at the region including the prior austenite grain boundaries are measured.
  • the distributions of concentration are analyzed by a ladder diagram to find the densities of segregated atoms per unit grain boundary area (interfacial excess).
  • the obtained values are made [B] ⁇ gb and [P] ⁇ gb .
  • [B] ⁇ gb and [P] ⁇ gb are measured for different prior austenite grain boundaries of five or more locations.
  • the average value of the values of the [B] ⁇ gb /[P] ⁇ gb obtained at the prior austenite grain boundaries are found.
  • the hot dip galvanized layer and hot dip galvannealed layer may be formed under usual plating conditions and alloying conditions. However, if the amount of Fe of the hot dip galvannealed layer is less than 7 mass %, the weldability and slidability cannot be secured, so the amount of Fe of the hot dip galvannealed layer is preferably 7 mass % or more.
  • the upper limit of the amount of Fe is 20 mass % or less from the viewpoint of suppression of powdering resistance, more preferably 15 mass % or less.
  • the amount of Fe of the hot dip galvannealed layer is adjusted by suitably adjusting the alloying conditions.
  • the uniform elongation in a direction perpendicular to the rolling direction is defined as “UE1” (uniform elongation)
  • the value of the total elongation (TE1 0 ) in a direction perpendicular to the rolling direction converted to the total elongation corresponding to a sheet thickness of 1.2 mm based on the following formula (1) is defined as “TE1” (total elongation)
  • the local elongation in a direction perpendicular to the rolling direction, corresponding to a sheet thickness 1.2 mm based on the following formula (2) is defined as “LE1” (local elongation).
  • TE 1 TE 1 0 ⁇ (1.2/ t 0 ) 0.2
  • LE 1 TE 1 ⁇ UE 1 (2)
  • the UE1 is the measured value of uniform elongation measured using a JIS No. 5 tensile test piece
  • TE1 0 is the measured value of the total elongation measured using a JIS No. 5 tensile test piece
  • t 0 is the sheet thickness of the JIS No. 5 tensile test piece used for measurement.
  • TE1 and LE1 are respectively the total elongation and local elongation converted to the case of a sheet thickness of 1.2 mm.
  • TS ⁇ UE1 becomes a large value if both of the tensile strength (TS) and uniform elongation (UE1) are excellent, so is used as an indicator for evaluating the uniform ductility.
  • TS ⁇ LE1 becomes a large value if both of the tensile strength (TS) and local elongation (LE1) are excellent, so is used as an indicator for evaluating the local ductility.
  • TS ⁇ UE1 is preferably 10000 MPa ⁇ % or more while TS ⁇ LE1 is preferably 5000 MPa ⁇ % or more. More preferably, TS ⁇ UE1 is 11000 MPa ⁇ % or more, while TS ⁇ LE1 is preferably 6000 MPa ⁇ % or more. Still more preferably TS ⁇ UE1 is 12000 MPa ⁇ % or more, while TS ⁇ LE1 is 7000 MPa ⁇ % or more.
  • the tensile strength (TS) is preferably 780 MPa or more, more preferably 980 MPa or more, still more preferably 1180 MPa or more.
  • the yield ratio (YR) is preferably 0.59 or more, more preferably 0.66 or more, still more preferably 0.72 or more.
  • TS ⁇ LE1 is preferably 5500 MPa ⁇ % or more, more preferably 6500 MPa ⁇ % or more.
  • a plurality of subsize Charpy impact test pieces having the directions perpendicular to the rolling direction as the length directions and having lengths of 55 mm, thicknesses of 10 mm, and widths of the sheet thicknesses of the steel sheets are stacked in the width direction and subjected to a Charpy impact test in that state.
  • the notch shapes of the test pieces are made the V-notches prescribed in JIS Z 2242.
  • the Charpy impact values when conducting the Charpy impact tests by test temperatures of ⁇ 60° C. and 40° C. are respectively defined as IV LT and IV HT .
  • IV LT /IV HT can be used as an indicator for evaluating the low temperature impact property.
  • IV LT /IV HT is preferably over 0.50, more preferably over 0.60, still more preferably over 0.70.
  • the steel sheet before plating of the present invention (below, referred to as the “base steel sheet”) need only be a steel sheet having the chemical composition of the steel sheet of the present invention.
  • the method for manufacturing the base steel sheet is not limited to a specific method of manufacture.
  • a hot rolled steel sheet can be used.
  • a cold rolled steel sheet obtained by pickling, then cold rolling a hot rolled steel sheet can also be used. Below, one example of a method for manufacturing a base steel sheet will be explained.
  • the method of casting the slab is not limited to any specific casting method, but the continuous casting method is preferable. It is also possible to use a steel ingot cast by another casting method as a steel slab by blooming etc. In the continuous casting process, to keep surface defects due to inclusions from forming, the molten steel is preferably made to flow inside the casting mold by electromagnetic stirring etc.
  • the steel ingot in the high temperature state after continuous casting or the steel slab in the high temperature state after blooming may be cooled once, then reheated and used for hot rolling.
  • the steel ingot in the high temperature state after continuous casting or the steel slab in the high temperature state after blooming may be used as is for hot rolling or may be used for hot rolling after auxiliary heating. Not that the steel ingot and steel slab used for the hot rolling will be generally referred to as a “slab”.
  • the temperature of the slab used for the hot rolling is preferably less than 1250° C. More preferably, it is 1200° C. or less.
  • the lower limit of the temperature of the slab used for the hot rolling is not particularly prescribed, but the temperature at which the hot rolling can be completed at the Ar 3 point or more is preferable.
  • the conditions of the hot rolling are not limited to specific conditions, but if the temperature of completion of the hot rolling is too low, in the metal structure of the hot rolled steel sheet, coarse low-temperature transformed structure stretched in the rolling direction are liable to be formed.
  • the temperature of completion of hot rolling is preferably the Ar 3 point or more and over 850° C. More preferably, it is the Ar 3 point or more and over 880° C., still more preferably the Ar 3 point or more and over 900° C.
  • the upper limit of the temperature of completion of hot rolling is not particularly prescribed, but from the viewpoint of refining the metal structure of the hot rolled steel sheet, 1000° C. or less is preferable.
  • the hot rolling is comprised of rough rolling and finish rolling
  • the rough rolled material may also be heated between the rough rolling and finish rolling.
  • the rough rolled material is heated so that the back end of the rough rolled material becomes a higher temperature than the front end of the rough rolled material and the variation in temperature over the entire length of the rough rolled material at the time of start of the finish rolling is controlled to 140° C. or less. Due to this temperature suppression, the uniformity of properties in the coil of the hot rolled steel sheet taken up is improved.
  • the rough rolled material may be heated using a known means.
  • a solenoid type induction heating apparatus may be provided between the rough rolling machine and finish rolling machine and the amount of temperature rise by heating by the solenoid type induction heating apparatus may be controlled based on the temperature distribution in the longitudinal direction of the rough rolled material at the upstream side of this induction heating apparatus etc.
  • the conditions from after the end of the hot rolling to the start of coiling may be the usual conditions, but by softening the hot rolled steel sheet, the cold rollability of the hot rolled steel sheet is improved.
  • the coiling temperature is preferably 600° C. or more.
  • the coiling temperature is more preferably 640° C. or more, more preferably 680° C. or more. If the coiling temperature is too high, the pickling ability of the hot rolled steel sheet falls, so the coiling temperature is preferably 750° C. or less, more preferably less than 720° C.
  • the conditions of the cold rolling are not limited to specific conditions.
  • a hot rolled steel sheet may be pickled or otherwise treated to remove scale.
  • the rolling reduction of the cold rolling is preferably 40% or more. If the rolling reduction is too high, the rolling load increases and rolling becomes difficult, so the rolling reduction is preferably less than 70%, more preferably less than 60%.
  • a base steel sheet is heated at a temperature over the Ac 1 point to anneal it.
  • the Ac 1 point is the temperature at which austenite starts to be formed in the metal structure when heating the base steel sheet.
  • the heating temperature is the Ac 1 point or more, austenite is not formed, retained austenite is not obtained in the metal structure of the steel sheet of the present invention, and the uniform ductility falls, so the heating temperature is preferably over the Ac 1 point. More preferably, it is over (Ac 1 +30)° C.
  • the heating temperature is preferably the (Ac 3 point-40)° C. or more. More preferably, it is over the Ac 3 point.
  • the Ac 3 point is the temperature at which ferrite disappears in the metal structure when heating a base steel sheet.
  • the heating temperature is preferably the (Ac 3 point+100)° C. or less, more preferably the (Ac 3 point+50)° C. or less.
  • the holding time at the heating temperature is not particularly limited, but to make the metal structure of the base steel sheet uniform, 10 seconds or more is preferable. From the viewpoint of coarsening of the austenite, 240 seconds or less is preferable.
  • the base steel sheet After annealing, the base steel sheet is cooled down to a temperature region of 500° C. or less by an average cooling rate in the temperature region of 650 to 500° C. of 2° C./s to less than 100° C./s without isothermal holding in the middle.
  • the cooling temperature region prescribing the average cooling rate is made a temperature region of 650 to 500° C. In this temperature region, ferrite and pearlite precipitate, so to adjust the amount of precipitation and secure the required mechanical properties, the cooling rate has to be controlled.
  • the average cooling rate in the temperature region of 650 to 500° C. is less than 2° C./s, polygonal ferrite and pearlite are excessively produced and the yield strength and tensile strength fall, so the average cooling rate in the above temperature region is preferably 2° C./s or more. More preferably, it is 4° C./s or more, more preferably 10° C./s or more.
  • the average cooling rate in the temperature region of 650 to 500° C. is 100° C./s or more, the precision of the shape and dimension of the steel sheet falls, so the average cooling rate in the above temperature region is preferably less than 100° C./s. More preferably, it is 30° C./s or less.
  • the base steel sheet is cooled to 500° C. or less by an average cooling rate in the temperature region of 650 to 500° C. of 2° C./s to less than 100° C./s.
  • the cooling conditions after cooling to 500° C. or less are not particularly limited, but in the metal structure after plating, it is preferable to adjust the vol % of the retained austenite and the amount of C of the retained austenite and, from the point of improving the uniform ductility and local ductility and raising the yield strength, it is preferable to hold the base steel sheet in the temperature region of 500° C. to 460° C. for 4 to 45 seconds.
  • the base steel sheet is hot dip galvanized in accordance with an ordinary method.
  • One surface or both surfaces of the base steel sheet are formed with a hot dip galvanized layer.
  • the base steel sheet may be suitably cooled and/or heated.
  • the bath temperature and bath composition of the hot dip galvanization may be the usual bath temperature and bath composition.
  • the amount of plating deposition may also be the usual amount of deposition. For example, a range of 20 to 80 g/m 2 per surface of the base steel sheet is preferable.
  • the base steel sheet having the hot dip galvanized layer may be heated to the required temperature to alloy the hot dip galvanized layer.
  • the alloying may be performed under usual conditions. For example, the alloying may be performed at 470 to 560° C. for 5 to 60 seconds. However, a condition where the amount of Fe in the plating layer becomes 7 mass % or more is preferable.
  • the plated or alloyed steel sheet is cooled down to less than 300° C. by an average cooling rate in the temperature from the plating temperature down to 300° C. and in the temperature region from the alloying temperature down to 300° C. of 2° C./s or more.
  • the average cooling rate is less than 2° C./s, pearlite is excessively formed, the yield strength and tensile strength fall, further, the amount of retained austenite decreases, and the uniform ductility falls, so the average cooling rate is preferably 2° C./s or more. More preferably, it is over 10° C./s.
  • the upper limit of the average cooling rate is not particularly limited, but from the viewpoint of economy, 500° C./s or less is preferable.
  • the cooling stopping temperature is less than 300° C., but from the viewpoint of effectively performing the later temper rolling, the cooling stopping temperature is preferably room temperature.
  • the sheet Before two-stage heat treatment of steel sheet having a hot dip galvanized layer or alloyed plating layer, the sheet is temper rolled by an elongation rate of 0.10% or more. Due to this temper rolling, in the subsequent two-stage heat treatment, the concentration of C at the austenite is promoted, the uniform ductility and local ductility are improved, and the yield strength is improved.
  • the elongation rate is less than 0.10%, in the subsequent two-stage heat treatment, concentration of C at the austenite is not promoted, the uniform ductility and local ductility are not improved, and, further, the yield strength is not improved, so the elongation rate is preferably 0.10% or more. More preferably, it is 0.30% or more, still more preferably 0.50% or more.
  • the upper limit of the elongation rate is not particularly prescribed, but if too high, the rolling load increases, so the elongation rate is preferably 2.00% or less. More preferably, it is less than 1.50%, still more preferably less than 1.00%.
  • the temper rolling temperature is not particularly limited, but from the point of effectively imparting work strain to the austenite, the lower the temperature, the better. Room temperature is particularly preferred.
  • Steel sheet having a hot dip galvanized layer or hot dip galvannealed layer is temper rolled by an elongation rate of 0.10% or more, then the steel sheet is heated up to 300° C. by an average heating rate of less than 10° C./s, then is heated in the over 300° C. to 600° C. temperature region by an average heating rate of 10° C./s or more and is held at the heating temperature in the over 300° C. to 600° C. temperature region for 1 second or more.
  • the ratio [C] ⁇ gb /[P] ⁇ gb of the amount of segregation of C (number of atoms/nm 2 ): [C] ⁇ gb to the amount of segregation of P (number of atoms/nm 2 ): [P] ⁇ gb at the prior austenite grain boundaries satisfies [C] ⁇ gb /[P] ⁇ gb ⁇ 4.0, C concentrates at the retained austenite and reaches 0.85 mass % or more, and the martensite is tempered. As a result, the uniform ductility and local ductility are improved, the yield strength is improved, and the low temperature impact property is improved.
  • the ratio [B] ⁇ gb /[P] ⁇ gb of the amount of segregation of B (number of atoms/nm 2 ): [B] ⁇ gb to the amount of segregation of P (number of atoms/nm 2 ): [P] ⁇ gb is satisfied [B] ⁇ gb /[P] ⁇ gb ⁇ 4.0, the uniform ductility and local ductility are improved, the yield strength is improved, and the low temperature impact property is improved.
  • C is made to concentrate at the austenite and martensite is tempered by heating the metal structure to an over 300° C. to 600° C. temperature region. At this time, up to 300° C., the heating is performed by a less than 10° C./s average heating rate. This heating promotes segregation of C and B at the prior austenite grain boundaries.
  • the average heating rate up to 300° C. is 10° C./s or more, segregation of C and B at the prior austenite grain boundaries is not promoted, so the average heating rate up to 300° C. is made less than 10° C./s. Preferably, it is 7° C./s or less, more preferably 3° C./s or less.
  • Average Heating Rate at Temperature Region of Over 300° C. to 600° C. 10° C./s or More
  • the average heating rate in the over 300° C. to 600° C. temperature region is preferably over 20° C./s. [C] ⁇ gb /[P] ⁇ g ⁇ 4.0 (3) [B] ⁇ gb /[P] ⁇ gb ⁇ 4.0 (4)
  • the steel sheet is held at a heating temperature of a temperature region of over 300° C. to 600° C. for 1 second or more. If the heating temperature is 300° C. or less, C insufficiently concentrates at the austenite, the uniform ductility is not improved, and, further, hard martensite remains and the local ductility is impaired and the yield strength falls, so the heating temperature is made over 300° C. Preferably it is over 350° C., more preferably over 400° C.
  • the heating temperature is made 600° C. or less.
  • the temperature is 550° C. or less, more preferably 500° C. or less.
  • the heating and holding time is made 1 second or more.
  • the time is 5 seconds or more, more preferably 15 seconds or more.
  • the heating and holding time is preferably 96 hours or less. More preferably, the time is 48 hours or less, still more preferably 24 hours or less.
  • the heating and holding time is suitably adjusted in accordance with the heating temperature.
  • the heating temperature is 400 to 600° C.
  • the heating and holding time is preferably 20 minutes or less. More preferably, it is 6 minutes or less, still more preferably less than 3 minutes. From the point of productivity, the heating temperature is preferably over 400° C. and the heating and holding time is preferably 20 minutes or less.
  • the steel sheet may be temper rolled to correct the flatness of the steel sheet. Further, the steel sheet may be formed with a coating oil or film for lubrication action.
  • the sheet thickness of the steel sheet of the present invention is not particularly limited to a specific range, but in the general use sheet thickness 0.8 to 2.3 mm steel sheets, the effect of the two-stage heat treatment remarkably appear.
  • a vacuum melting furnace was used to cast molten steel to produce Steels A to U having the chemical compositions shown in Table 1.
  • the Ac 1 point and Ac 3 point in Table 1 were found from the changes in heat expansion when heating cold rolled steel sheets of Steels A to P by 2° C./s.
  • the Steels A to U were heated to 1200° C. and held there for 60 minutes, then were hot rolled under the hot rolling conditions shown in Tables 2-1 and 2-2.
  • the Steels A to U are rolled by 10 passes to obtain thickness 2.5 to 3.0 mm hot rolled steel sheets.
  • water spray was used to cool the hot rolled steel sheets to 500 to 680° C.
  • the cooling end temperature was made the coiling temperature
  • the hot rolled steel sheets were charged into the electric heating furnace held at that coiling temperature and held there for 60 minutes, then the hot rolled steel sheets were furnace cooled by a 20° C./hr cooling rate down to room temperature to simulate gradual cooling after coiling.
  • the hot rolled steel sheets were pickled to obtain the base materials for cold rolling. These were cold rolled by rolling reductions of 47 to 52% to obtain thickness 1.2 to 1.6 mm cold rolled steel sheets (base steel sheets).
  • a hot dip galvanization simulator was used to heat the base steel sheets by a 10° C./s heating rate up to 650° C., then heat them by a 2° C./s heating rate to the temperatures shown in Tables 2-1 and 2-2 and soak them for 30 to 90 seconds.
  • the base steel sheets were cooled under the cooling conditions shown in Tables 2-1 and 2-2 down to 460° C. then the base steel sheets were dipped in a hot dip galvanization bath held at 460° C. to hot dip galvanize the base steel sheets. Some of the base steel sheets were heated up to 520° C. to alloy them after the hot dip galvanization.
  • the base steel sheets were secondarily cooled under the cooling conditions shown in Tables 2-1 and 2-2.
  • “RT” shows room temperature.
  • the secondarily cooled base steel sheets were temper rolled by a 0.50% elongation rate, then were heat treated under the heat treatment conditions shown in Tables 3-1 and 3-2 to obtain hot dip galvanized steel sheet or hot dip galvannealed steel sheet (below, hot dip galvanized steel sheet and hot dip galvannealed steel sheet referred to all together as “plated steel sheet”.)
  • temper rolling was performed without cooling down to room temperature after stopping the secondary cooling, then heat treatment was performed under the heat treatment conditions shown in Tables 3-1 and 3-2 without cooling down to room temperature. For some of the base steel sheets, the temper rolling or heat treatment was omitted. In the “Heat treatment conditions” column of Tables 3-1 and 3-2, “-” indicates no heat treatment performed.
  • An XRD measurement use test piece was taken from a plated steel sheet and the rolled surface of the test piece was chemical-polished down to a depth position of 1 ⁇ 4 of the sheet thickness from the boundary of the base steel sheet and plating layer.
  • the rolled surface was subjected to an X-ray diffraction test to measure the volume ratio of the retained austenite and the amount of C of the retained austenite.
  • test piece was irradiated with Mo—K ⁇ rays to measure the integrated strengths of the ⁇ phase (200), (211) diffraction peaks and the integrated strength of the ⁇ phase (200), (220), (311) diffraction peaks and find the volume ratio of retained austenite.
  • an EBSP measurement use test piece was taken from a plated steel sheet, a longitudinal section parallel to the rolling direction was electrolytic-polished, then the metal structure at a depth position of 1 ⁇ 4 of the sheet thickness of the base steel sheet from the boundary of the base steel sheet and plating layer was observed and the prior austenite grain boundaries were confirmed.
  • FIB was used to fabricate a 3DAP measurement use pin sample including prior austenite grain boundaries.
  • a 3DAP was used to measure the distributions of concentrations of C, B, and P atoms, find the amount of segregation of C ([C] ⁇ gb ), the amount of segregation of B ([B] ⁇ gb ), and the amount of segregation of P ([P] ⁇ gb ) at the prior austenite grain boundaries, and calculate [C] ⁇ gb /[P] ⁇ gb and [B] ⁇ gb /[P] ⁇ gb .
  • test piece for SEM observation use was taken from a plated steel sheet, a longitudinal section of the test piece parallel to the rolling direction was polished, then this longitudinal section was corroded by Nital corrosion and LePera corrosion and the metal structure at a depth position of 1 ⁇ 4 of the sheet thickness of the base steel sheet from the boundary of the base steel sheet and plating layer was observed.
  • Image processing was used to measure the volume ratios of the tempered martensite, polygonal ferrite, fresh martensite, and remaining structure.
  • the volume ratio of the fresh martensite was found by subtracting the volume ratio of the retained austenite measured by the above XRD measurement from the total of the retained austenite and fresh martensite measured by LePera corrosion.
  • the yield stress (YS), tensile strength (TS), and uniform elongation (UE1) were found by taking from the plated steel sheets JIS No. 5 tensile test pieces along a direction perpendicular to the rolling direction and performing tensile tests on the test pieces.
  • the tensile rate was made 1 mm/min until reaching the yield point and then 10 mm/min.
  • the yield ratio (YR) was found by dividing YS by TS.
  • the total elongation (TE1) and the local elongation (LE1) are values obtained by taking JIS No. 5 tensile test pieces along the direction perpendicular to the rolling direction, conducting tensile tests, and using the measured value of the total elongation (TE1 0 ) and the measured value of the uniform elongation (UE1) to find converted values corresponding to the case of the sheet thickness of 1.2 mm based on the above formulas (1) and (2).
  • V-notch subsize Charpy impact test pieces were taken from the plated steel sheets in a direction perpendicular to the rolling direction. If the sheet thickness was 1.2 mm, eight sheets were stacked screwed together and if the sheet thickness was 1.6 mm, six sheets were stacked and screwed together. These test pieces were used for performing Charpy impact tests.
  • the Charpy impact value in the case of the test temperature of ⁇ 60° C. was designated as IV LT
  • the Charpy impact value in the case of the test temperature of 40° C. was designated as IV HT
  • the value of IV LT /IV HT was found.
  • TS ⁇ UE1 was 10000 MPa ⁇ % or more, and TS ⁇ LE1 was 5000 MPa ⁇ % or more, it was judged that the mechanical properties were excellent. Further, if IV LT /IV HT was over 0.50, it was judged that the low temperature impact property was excellent.
  • Tables 4-1 and 4-2 show the results of examination of the metal structures of the plated steel sheets, while Tables 5-1 and 5-2 show the results of evaluation of the mechanical properties and low temperature impact property of the plated steel sheets.
  • Metal structure of plated steel sheet column: “[C] ⁇ gb/[P] ⁇ gb” indicates ratio of amount of segregation of C at prior austenite grain boundaries to amount of segregation of P at prior austenite grain boundaries, while “[B] ⁇ gb/[P] ⁇ gb” indicates ratio of amount of segregation of B at prior austenite grain boundaries to amount of segregation of P at prior austenite grain boundaries. 2.
  • metal structure of plated steel sheet “Amount of C of retained austenite” column, “[C] ⁇ gb/[P] ⁇ gb” column, and “[B] ⁇ gb/[P] ⁇ gb” column: “—” shows these measurements not performed.
  • the invention examples all exhibited a TS ⁇ UE1 of 10000 or more, a TS ⁇ LE1 of 5000 or more, and excellent uniform ductility and local ductility. Further, YR exhibited a high value of 0.59 or more. Further, IV LT /IV HT exhibited a high value of 0.51 or more.
  • test results for the comparative examples where the chemical composition or steps and conditions were not suitable were all inferior in part or all of the yield ratio, uniform ductility, local ductility, and low temperature impact property.
  • the heat treatment temperature is too high, so the volume ratio of the retained austenite, the amount of C of the retained austenite, and [C] ⁇ gb /[P] ⁇ gb are low and YR, TS ⁇ UE1, TS ⁇ LE1, and IV LT /IV HT are low.
  • the average cooling rate in the temperature region of 650 to 500° C. in the first cooling step is too low.
  • the volume ratio of the retained austenite and the volume ratio of the tempered martensite are low and the YR and TS ⁇ LE1 are low.
  • the volume ratio of the retained austenite is low and YR, TS ⁇ UE1, and TS ⁇ LE1 are low.
  • hot dip galvanized steel sheet and hot dip galvannealed steel sheet which are excellent in both uniform ductility and local ductility, excellent in press formability, and high in yield strength and tensile strength and which are excellent in local ductility, excellent in impact adsorption, and excellent in low temperature impact property.
  • the hot dip galvanized steel sheet and hot dip galvannealed steel sheet of the present invention are steel sheets optimal as base steel sheets of member or pillars or other structural parts of auto bodies or other machine structure parts. Therefore, the present invention is high in applicability in the automotive industry and machine part manufacturing industry.

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US20220090223A1 (en) * 2018-12-26 2022-03-24 Jfe Steel Corporation High-strength hot-dip galvanized steel sheet and method for manufacturing the same
US11905570B2 (en) * 2019-02-06 2024-02-20 Nippon Steel Corporation Hot dip galvanized steel sheet and method for producing same
CN112538593B (zh) * 2020-11-09 2022-06-10 鞍钢蒂森克虏伯汽车钢有限公司 一种控制表面波纹度的热镀锌if钢板生产方法
CN115181884B (zh) * 2021-04-02 2023-08-11 宝山钢铁股份有限公司 1280MPa级别低碳低合金热镀锌Q&P钢及快速热处理热镀锌制造方法
JP7260073B1 (ja) 2021-08-30 2023-04-18 Ｊｆｅスチール株式会社 高強度鋼板，高強度めっき鋼板及びそれらの製造方法，並びに部材
WO2023095870A1 (ja) * 2021-11-26 2023-06-01 日本製鉄株式会社 亜鉛めっき鋼板
CN114990425B (zh) * 2022-01-11 2023-07-18 长沙中金智能装备有限公司 一种废钢破碎用刀具及其制备、修复方法

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